1. Field of the Invention
This invention relates to a marine drive transmission, and more particularly to a relatively compact marine drive transmission that smoothly shifts into both the forward or reverse drive conditions.
2. Description of Related Art
A wide variety of marine propulsion units propel watercrafts. For instance, outboard motors commonly power boats and other watercraft. Stem drive units, which include an inboard motor and an outboard drive, also are often used to power boats and watercraft.
An outboard motor conventionally includes a power head at the top of the drive unit. The power head includes an internal combustion engine having an output shaft extending generally vertically. A driveshaft housing of the drive unit depends from the power head and encloses a driveshaft that extends generally vertically from the output shaft. A lower unit further depends from the driveshaft housing. A propeller shaft is provided therein and extends generally horizontally. The driveshaft and the propeller shaft are connected in the lower unit so that the propeller shaft extends normal to the driveshaft. A propulsion device such as, for example, a propeller is affixed to an outer end of the propeller shaft. A bevel gear transmission, for example, is provided between the driveshaft and the propeller shaft that includes a forward, neutral, reverse shift mechanism for switching over to one of forward, neutral and reverse positions from another position. The engine power is transmitted to the propeller through the output shaft, driveshaft, bevel gear transmission and propeller shaft. The propeller, thus, can propel the outboard motor and the associated watercraft in both forward and reverse directions, if the shift mechanism is not in the neutral position.
An outboard section of the stem drive unit has a construction similar to that of the outboard motor except that it has no engine atop thereof. The engine is placed in the hull of the associated watercraft. A propulsion device of the stem drive unit, which typically is a propeller, is powered by the engine through the driveshaft and propeller shaft combination (i.e., drive train arrangement) similar to that of the drive unit of the outboard motor.
Consumers continue to desire more powerful marine drives and prefer large propulsion units having engines which produce high horsepower. An engine, for example, which operates on a four stroke principle and having a plurality of cylinders, can provide the desired increased horsepower.
However, due to carrying such a large engine, the marine propulsion unit tends to jolt the occupants of the watercraft when the shift mechanism is operated and thereby gives the occupants an uncomfortable feeling. That is, since the large-sized engine generates a relatively large propulsion force, it gives rise an uncomfortable shock to the occupants by abrupt change of the propulsion force particularly when the shift mechanism is shifted from the neutral position to the forward drive position or to the reverse drive position.
In order to address this problem, a smoothing device for the shifting operation has been proposed in U.S. Pat. No. 4,747,796. FIGS. 1 and 2 illustrate this type of coupling and correspond to FIGS. 11 and 12 of U.S. Pat. No. 4,747,796. FIGS. 1 illustrates a cross-sectional, side elevational view of a conventional coupling 20 arranged to absorb the shock, and FIG. 2 illustrates a cross-sectional view of the same coupling 20 taken along the line 2--2 in FIG. 1.
With reference to these figures, a driveshaft 22 is divided into a drive section 24 and a driven section 26 and the coupling 20 is provided therebetween to couple them. The lower end of the drive section 24 has a depending flange 28 that defines an internal cavity 30. The upper end of the driven section 26 has a projecting portion 32 that extends into the cavity 30. Three blocks of elastic members 34 are interposed between the internal cavity 30 and the projecting portion 32. As seen in FIG. 2, the flange 28 and its internal cavity 30 have a generally triangular configuration. The projecting portion 32 is also triangular in shape with three apices 38.
The coupling 20 provides vibration damping and force absorption under low speed and load conditions. This damping is provided by the elastic members 34 that are compressible by certain compressive force exerted thereupon. When the driving loads are increased, the elastic members 34 are extremely compressed and the apices 38 of the projecting portion 32 directly contact inner cavity 30 of the flange 28 of the drive portion 24. The torque of the drive section 24 is transmitted to the driven section 26 by this connection.
Because the transmission shifting shock occurs under the low speed condition, the coupling 20 is quite useful for preventing the shock from occurring when the shift mechanism is shifted. However, another problem appears with this coupling 20. The problem is that the driving force is not securely transferred from the drive section 24 to the driven section 26 when the driving loads increase. Because the driving force is conveyed by the contacts of the apices 38 with the inner cavity 30 and these contacts are not so reliable. Of course, the elastic members 34 are also involved in this force transferring mechanism. However, the elastic members 34 are slippery in the cavity 30 and do not increase reliability.